The presence of sulfur in gases is undesirable because of its noxious odor, toxicity, and corrosive properties. Furthermore, sulfur is undesirable because it is a source of atmospheric pollution. The processes whereby the sulfur is removed from the gas are generically known as "sweetening".
Removal of sulfur compounds from gas streams has been of considerable importance in the past and is even more important today due to environmental considerations.
Numerous natural gas wells produce what is known as "sour gas". Sour gas is natural gas that contains sulfur compounds such as hydrogen sulfide. Considerable effort has been expended to find effective and cost efficient means of removing these sulfur compounds from natural gas.
Gas a effluent from the combustion of organic materials, such as oil and coal, also usually contains sulfur compounds. With increasing emphasis on the elimination of sulfur discharged to the atmosphere it has become very desirable to remove the sulfur compounds from the effluent.
Sulfur is also present in many gas streams which are the result of petroleum refining and manufacturing.
These gases have often been treated by using various absorption-desorption processes. Illustrative absorption-desorption processes are the hot potassium carbonate process, the vacuum carbonate process, the amine process (especially those utilizing mono-, di-, and triethanolamine), and various other processes using organic solvents. However, these processes, while effective in reducing the sulfur content to a low level, require the gases recovered from the process to be further treated to lower the sulfur content to a satisfactory level.
Furthermore, these processes are generally carried out at elevated temperatures and pressures which increase operating expenses and the dangers of utilizing these processes. For example, the amine process utilizes a temperature of about 40.degree. C. in the absorption stage and about 120.degree. C. in the solvent regeneration stage. The hot potassium carbonate process utilizes a pressure in excess of 250 psi.
Frequently, these recovered gases from the absorption step have been used to produce elemental sulfur by some variation of the Claus process. In this process a portion of the sulfur removed, usually in the form of hydrogen sulfide, is oxidized to sulfur dioxide and this sulfur dioxide and the remaining hydrogen sulfide are reacted in a catalytic converter to form elemental sulfur and water.
The Claus and related processes for recovering elemental sulfur have a disadvantage in that there is invariably a residue of gas, known as the tail gas, in which either sulfur dioxide or hydrogen sulfide, or frequently both, remain. This tail gas is usually discharged into the atmosphere which is highly undesirable as this results in pollution of the environment.
Alternatively, the tail gas or other gases are processed by catalytic oxidation of hydrogen sulfide with oxygen as illustrated in U.S. Pat. No. 4,311,683 to Hass et al. The sulfur thus removed is in the form of elemental sulfur. However, this process requires an elevated temperature of about 120.degree. to about 235.degree. C. (about 250.degree. to about 450.degree. F.) and a catalyst, both of which are undesirable.
The reaction between hydrogen sulfide and ozone in air has been described by Hales et al., Atmospheric Environment, Vol. 3, Pergamon Press, London, Great Britain (1969), pp. 657-667.
While there are many industrial uses for sulfur, one of the most important uses is in sulfate-containing compounds suitable for use as agricultural fertilizers. Ammonium sulfate is a desirable fertilizer. Unfortunately, with the prior processes, the elemental sulfur must be further processed to produce the sulfate-containing compound.
A process wherein the amount of sulfur present in a feed gas is significantly reduced and the sulfur thus obtained is in a commercially useable form without requiring further processing is very desirable. It is also desirable to operate the process at approximately ambient temperature, at a relatively low pressure and without a catalyst. These desires are satisfied by the present invention.